Portable appliance motor control with speed-based current limitation

ABSTRACT

A method of controlling a portable appliance includes measuring an input current supplied to a motor of the portable appliance and measuring a rotational speed of a shaft of the motor. The method also includes determining a current limit based on the rotational speed of the shaft using a substantially continuous function which relates a domain of rotational speeds to a range of current limits. The method further includes reducing, when the input current exceeds the current limit, the rotational speed of the shaft incrementally along the substantially continuous function until the input current is approximately equal to the current limit.

CROSS-REFERENCE TO RELATE APPLICATION

This application is a continuation of U.S. patent application Ser. No.13/882,203, filed Apr. 29, 2013, and entitled “PORTABLE APPLIANCE MOTORCONTROL WITH SPEED-BASED CURRENT LIMITATION,” which is a National StageEntry of PCT/CN2010/001750. The entire disclosures of theabove-identified applications are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates generally to portable appliances, such asstand mixers. The present disclosure relates more particularly to amethod of controlling a motor of a portable appliance in which a currentlimit is determined based on a rotational speed of a shaft of the motor.

BACKGROUND ART

A portable appliance, or small appliance, is a device which may be usedin the preparation of meals and other foodstuffs. Typically, portableappliances are intended to be used in a handheld manner or on tabletops,countertops, or other platforms. Many portable appliances include amotor and electronics to control the operation of the motor.Illustrative examples of such portable appliances include stand mixers,hand mixers, blenders, immersion blenders, juicers, and food processors.

DISCLOSURE OF INVENTION

According to one aspect of this disclosure, a method of controlling aportable appliance includes measuring an input current supplied to amotor of the portable appliance and measuring a rotational speed of ashaft of the motor. The method also includes determining a current limitbased on the rotational speed of the shaft using a substantiallycontinuous function which relates a domain of rotational speeds to arange of current limits. The method further includes reducing, when theinput current exceeds the current limit, the rotational speed of theshaft incrementally along the substantially continuous function untilthe input current is approximately equal to the current limit.

In some embodiments, measuring the input current supplied to the motormay include periodically sampling, at a first sampling rate, an outputsignal of a current sensor. Measuring the rotational speed of the shaftof the motor may include periodically sampling, at a second samplingrate, an output signal of an RPM sensor. The first and second samplingrates each may have a greater frequency than a periodic drive signalused to drive the motor.

In some embodiments, determining the current limit using thesubstantially continuous function may include calculating the output ofa linear function with the rotational speed of the shaft as the input.In other embodiments, determining the current limit using thesubstantially continuous function may include calculating the output ofa non-linear function with the rotational speed of the shaft as theinput. In still other embodiments, determining the current limit usingthe substantially continuous function may include retrieving a valuewhich corresponds to the rotational speed of the shaft from a look-uptable.

In other embodiments, reducing the rotational speed of the shaftincrementally along the substantially continuous function may comprise,iteratively, (i) reducing the rotational speed of the shaft by anincrement, (ii) measuring a new rotational speed of the shaft, (iii)measuring a new input current, and (iv) determining a new current limitbased on the new rotational speed of the shaft using the substantiallycontinuous function, until the new input current is approximately equalto the new current limit. In such embodiments, each iteration of steps(i)-(iv) may be performed within a period of a periodic drive signalused to drive the motor.

In some embodiments, the method may further include operating the motorusing a proportional-integral-derivative (PID) algorithm to maintain therotational speed of the shaft at a desired speed setting, until theinput current equals the current limit. In such embodiments, the methodmay further include incrementally increasing a previously reducedrotational speed of the shaft, when the input current is less than thecurrent limit and until the rotational speed of the shaft isapproximately equal to the desired speed setting.

According to another aspect, a mixer includes a user control operable togenerate an input signal indicative of a desired speed setting for themixer and a motor having a shaft configured to provide motive power to amixing element. The mixer also includes a current sensor operable togenerate a current signal indicative of an input current supplied to themotor and an RPM sensor operable to generate a speed signal indicativeof a rotational speed of the shaft of the motor. The mixer furtherincludes an electronic controller operable to (i) generate a motorcontrol signal such that the rotational speed of the shaft correspondsto the desired speed setting, (ii) calculate a current limit based onthe speed signal using a substantially continuous function which relatesa domain of rotational speeds of the shaft to a range of current limits,and (iii) modify the motor control signal, when the current signalexceeds the current limit, such that the rotational speed of the shaftis incrementally reduced along the substantially continuous functionuntil the current signal is approximately equal to the current limit.

In some embodiments, the current signal exceeding the current limit maybe associated with a pinch point between the mixing element and a mixerbowl that receives the mixing element. The substantially continuousfunction may comprise one of a linear function, non-linear function, anda look-up table. The mixer may further include a driver circuit operableto generate a periodic drive signal to drive the motor in response tothe motor control signal. The electronic controller may further beoperable to, at least once during each period of the periodic drivesignal, (i) calculate a new current limit based on the speed signalusing the substantially continuous function and (ii) modify the motorcontrol signal, when the current signal exceeds the new current limit,such that an average current of the periodic drive signal isincrementally reduced.

According to yet another aspect, a tangible, machine readable mediumcomprises a plurality of instructions that, in response to beingexecuted, result in an electronic controller receiving an input signalindicative of a desired speed setting for a portable appliance having amotor, generating a motor control signal such that a rotational speed ofa shaft of the motor corresponds to the desired speed setting, receivinga speed signal indicative of the rotational speed of the shaft,receiving a current signal indicative of an input current supplied tothe motor, calculating a current limit based on the speed signal using asubstantially continuous function which relates a domain of rotationalspeeds of the shaft to a range of current limits, and modifying themotor control signal, when the current signal exceeds the current limit,such that the rotational speed of the shaft is incrementally reducedalong the substantially continuous function until the current signal isapproximately equal to the current limit.

In some embodiments, the plurality of instructions, in response to beingexecuted, may further result in the electronic controller modifying themotor control signal using a proportional-integral-derivative (PID)algorithm to maintain the rotational speed of the shaft at the desiredspeed setting, until the current signal equals the current limit. Inother embodiments, using the substantially continuous function maycomprise using one of a linear function, non-linear function, and alook-up table.

In other embodiments, modifying the motor control signal may include,iteratively, (i) generating a modified motor control signal such thatthe rotational speed of the shaft is reduced by an increment, (ii)calculating a new current limit based on the speed signal using thesubstantially continuous function, until the current signal isapproximately equal to the new current limit. In such embodiments, eachiteration of steps (i)-(ii) may be performed within a period of aperiodic drive signal used to drive the motor. The plurality ofinstructions, in response to being executed, may further result in theelectronic controller modifying the motor control signal such that therotational speed of the shaft is incrementally increased along thesubstantially continuous function, when the current signal is less thanthe current limit and until the rotational speed of the shaftcorresponds to the desired speed setting.

BRIEF DESCRIPTION OF DRAWINGS

The detailed description particularly refers to the following figures,in which:

FIG. 1 is a perspective view of a portable appliance, embodied as astand mixer having a dough hook;

FIG. 2 is a schematic representation illustrating a complex rotationalmotion of the dough hook of FIG. 1;

FIG. 3 is a block diagram illustrating a motor and associated motorcontrols of the stand mixer of FIG. 1;

FIG. 4 is flowchart illustrating an operational algorithm of the motorcontrols of FIG. 3; and

FIG. 5 is a graph illustrating various settings and currents of themotor of FIG. 3 when operated according to the algorithm of FIG. 4.

BEST MODE(S) FOR CARRYING OUT THE INVENTION

While the concepts of the present disclosure are susceptible to variousmodifications and alternative forms, specific exemplary embodimentsthereof have been shown by way of example in the drawings and willherein be described in detail. It should be understood, however, thatthere is no intent to limit the concepts of the present disclosure tothe particular forms disclosed, but on the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

A portable appliance 10, illustratively embodied as a stand mixer 10, isshown in FIG. 1. The stand mixer 10 has a mixer head 12 and a base 14having an upstanding pedestal portion 16 supporting the mixer head 12.The mixer head 12 encases a motor 18 and associated electronic motorcontrols 20, which are shown in phantom. In some embodiments, the motor18 and/or the motor controls 20 may alternatively be located in the base14, including the upstanding pedestal portion 16.

A user control 22 is also included on the stand mixer 10. The usercontrol 22 is illustratively embodied in FIG. 1 as a sliding speedcontrol knob 22 mounted in the mixer head 12. The user may choose adesired speed setting with the control knob 22, and the motor controls20 will generally attempt to operate the motor 18 at the desired speed.As shown in FIG. 1, the sliding speed control knob 22 is configured formultiple discrete speeds, indexed from 0 to 10, with an increment of 1.It will be appreciated that in other embodiments the user control 22 maybe any type of analog or digital user interface operable to input adesired speed setting for the stand mixer 10.

The stand mixer 10 includes several mixing elements 24 which may bereleasably attached to the mixer head 12 for rotation thereby. A mixingelement 24 embodied as a dough hook 24 is shown (partially in phantom)in FIG. 1. Other possible mixing elements include a wire whip, a flatbeater, and the like. These mixing elements 24 mix foodstuffs and otheritems in a mixer bowl 26, which is supported on the base 14. In theillustrative embodiment, the motor 18 is configured to provide motivepower to the mixing element 24 via a planetary gear system. Exemplaryplanetary gear systems and their operation are described in U.S. patentapplication Ser. Nos. 11/930,900 (entitled “Utilizing Motor CurrentVariations to Control Mixer Operation”) and Ser. No. 11/931,114(entitled “Smoothing Motor Speed During Mixing”), both of which werefiled Oct. 31, 2007, are assigned to the assignee of the presentapplication, and are expressly incorporated by reference herein in theirentirety. As described therein, the use of the planetary gear systemcreates a complex rotational motion for the mixing element 24 (e.g., thedough hook 24) because the mixing element 24 both orbits around the axisof rotation of a sun gear and rotates along the axis of rotation of aplanetary gear to which it is coupled.

The complex rotational motion of the dough hook 24 within the mixer bowl26 is illustrated in FIG. 2. The path taken by a shaft of the dough hook24 is represented by line 30. Portions of the path taken by an outermostpoint of the dough hook 24 are represented by lines 32, 34. For example,the movement of the outermost portion of the dough hook 24 during afirst full planetary gear rotation is illustrated by line 32, while themovement of the outermost portion of the dough hook 24 during a secondfull planetary gear rotation is illustrated (in phantom) by line 34.Thus, the path taken by the outermost point of the dough hook 24advances relative to the mixer bowl 26 for each orbit about the sungear. In other words, the path of the outermost point of the dough hook24 is not repetitive or fixed relative to the mixer bowl 26.

As can been seen in FIG. 2, this complex rotational movement createsseveral pinch points 36, 37, 38, 39, 40, 41 where the distance betweenthe outermost point of the dough hook 24 and the mixer bowl 26 is at aminimum. It will be appreciated that the locations of the pinch points36-41 is a physical feature that will vary depending on the particularstand mixer 10. During all phases of the rotation of the dough hook,there is an intermittent grabbing and slipping of foodstuffs (e.g.,dough) with respect to the dough hook 24 and the mixer bowl 26 thatresults in an intermittent application of the weight of the foodstuff tothe motor 18 (i.e., an intermittent or instantaneous loading andunloading of the motor 18). The pinch points 36-41, however, representthe times of highest loading for the motor 18. For example, as theoutermost point of the dough hook 24 passes through points 42, 44 andnears the pinch point 36, the load on the motor 18 will graduallyincrease. Conversely, after the outermost point of the dough hook 24passes the pinch point 36 and subsequently passes through points 46, 48,the load on the motor 18 will gradually decrease. It will also beappreciated that the timing, magnitude, and frequency of theintermittent or instantaneous loading of the motor 18 may be impacted byfactors other than the location of the pinch points 36-41, such as thetype of foodstuff being mixed, by way of example.

Referring now to FIG. 3, the motor 18 and associated motor controls 20,according to one illustrative embodiment, are shown as a simplifiedblock diagram. These components are labeled using the same referencenumerals as FIG. 1, and similar components are labeled using similarreference numerals in all figures throughout this disclosure. The motorcontrols 20 may include an electronic control unit (ECU) or “electroniccontroller” 50, a memory device 52, a driver circuit 54, a currentsensor 56, and a comparator 58. In some embodiments, the user control 22may also be incorporated into motor controls 20. In other embodiments(such as that shown in FIG. 3), the user control 22 is separated fromthe motor controls 20 but provides an input signal to the electroniccontroller 50. It should also be appreciated that some components of themotor controls 20 (e.g., a power supply circuit) are not shown in FIG. 3for the sake of clarity.

As previously mentioned, the stand mixer 10 utilizes a motor 18 toprovide motive power to the mixing element 24. The motor 18 mayillustratively be embodied as a brushed, brushless, or stepper directcurrent (DC) motor, a universal motor, or the like. The motor 18includes a shaft 60 which revolves about its axis when power is suppliedto the motor 18. This shaft 60 may be characterized by its rotationalspeed, or angular velocity. The shaft 60, in turn, transfers its motionto a mixing element 24, either directly or indirectly via a transmission(e.g., via the planetary gear system in the illustrative embodiment).Interactions between the mixing element 24 and foodstuffs in the mixerbowl 26 may subject the motor 18 and the transmission to intermittent orinstantaneous loading, as described above.

The rotational speed of the shaft 60 of motor 18 may be measured by anRPM (revolutions-per-minute) sensor 62. Any type of sensor capable ofmeasuring rotational speed or angular velocity may be used as the RPMsensor 62. The RPM sensor 62 may illustratively be embodied as aHall-effect sensor 62 in cases where the motor 18 includes a ring magnetwhich revolves with the shaft 60. A Hall-effect sensor 62 responds tochanges in magnetic fields within its proximity by altering themagnitude of an output voltage. Thus, the Hall-effect sensor 62 willgenerate an output signal indicative of a rotational speed of the shaft60 of motor 18. In some embodiments, the output signal of sensor 62 maybe an analog voltage that represents the rotational frequency of shaft60 (for example, in Hertz). As shown in FIG. 3, this output signal isprovided to the electronic controller 50.

The motor 18 of FIG. 3 is driven by a periodic drive signal that isgenerated by the driver circuit 54. The periodic drive signal may beillustratively embodied as a pulse-width-modulation (PWM) signal havinga constant frequency and amplitude, but a variable duty cycle (i.e., “ontime” versus “off time”). In such embodiments, the driver circuit 54receives a motor control signal from the electronic controller 50 andgenerates a PWM signal of a particular duty cycle in response to themotor control signal. To generate the PWM signal, the driver circuit 54may switch a field-effect transistor (FET) in response to the motorcontrol signal. As the duty cycle of the PWM signal increases, largeraverage currents will be delivered to the motor 18, and the shaft 60will operate at higher rotational speeds (when torque is constant). Inother embodiments, the driver circuit 54 may instead include a triac(“triode for alternating current,” also known as a bidirectional triodethyristor) configured to generate a generally sinusoidal periodic drivesignal. In such embodiments, a motor control signal from the electroniccontroller 50 may trigger the triac at different phase angles duringeach half-cycle of an AC supply voltage, resulting in varying averagecurrent values for the periodic drive signal. It will be appreciated bythose of skill in the art that many types of driver circuits andperiodic drive signals are possible.

The input current supplied to the motor 18 is measured by a currentsensor 56. Any type of sensor capable of measuring an electrical currentmay be used as the current sensor 56. The current sensor 56 mayillustratively be embodied as an amplifier that measures the voltageacross a small resistor coupled in series with the motor 18. The voltageoutput of this amplifier will be proportional to the input currentsupplied to the motor 18. Thus, the current sensor 56 will generate anoutput signal indicative of an input current supplied to the motor 18.As shown in FIG. 3, this output signal is provided to the electroniccontroller 50. In other embodiments, the current sensor 56 mayinductively sense the input current supplied to the motor 18.

The output signal of the current sensor 56 is also provided as one ofthe two inputs of a comparator 58. The other input of the comparator 58is a reference voltage V_(ref). The comparator 58 has a digital, orbinary, output signal representing whether the input current of themotor 18 has exceeded a predetermined threshold (represented byV_(ref)). In some embodiments, the voltage comparator 58 may beillustratively embodied as an amplifier that outputs a “low” signal whenthe magnitude of the voltage provided by the current sensor 56 is lessthan the magnitude of V_(ref) and that outputs a “high” signal when themagnitude of the voltage provided by the current sensor 56 is greaterthan the magnitude of V_(ref). The magnitude of the reference voltageV_(ref) may be adjusted by the electronic controller 50 to set variousthresholds.

The motor controls 20 also include an electronic controller 50. Theelectronic controller 50 is, in essence, the master computer responsiblefor interpreting electrical signals sent by sensors associated with thestand mixer 10 and for activating or energizingelectronically-controlled components associated with the stand mixer 10.For example, the electronic controller 50 is configured to controloperation of the motor 18, to monitor various signals from the usercontrol 22, the current sensor 56, the comparator 58, and the RPM sensor62, and to determine whether the stand mixer 10 should operate in acurrent limitation mode, amongst many other things. In particular, aswill be described in more detail below with reference to FIGS. 4 and 5,the electronic controller 50 is operable to generate a motor controlsignal such that the rotational speed of the shaft 60 corresponds to adesired speed setting, calculate a current limit based on a speed signalfrom the RPM sensor 62 using a substantially continuous function whichrelates a domain of rotational speeds of the shaft to a range of currentlimits, and modify the motor control signal, when a current signal fromthe current sensor 56 exceeds the current limit, such that therotational speed of the shaft 60 is incrementally reduced along thesubstantially continuous function until the current signal isapproximately equal to the current limit.

To do so, the electronic controller 50 includes a number of electroniccomponents commonly associated with electronic units utilized in thecontrol of electromechanical systems. For example, the electroniccontroller 50 may include, amongst other components customarily includedin such devices, a processor, such as a microprocessor. Themicroprocessor of the electronic controller 50 may interface with amemory device 52, such as a programmable read-only memory device(“PROM”), including erasable PROM's (EPROM's or EEPROM's). In someembodiments, the memory device 52 may be a component of the electroniccontroller 50. The memory device 52 is provided to store, amongst otherthings, instructions in the form of, for example, a software routine (orroutines) which, when executed by the microprocessor, allows theelectronic controller 50 to control operation of the stand mixer 10.

The electronic controller 50 may also include an analog interfacecircuit. The analog interface circuit converts the output signals fromvarious sensors (e.g., the RPM sensor 62) into signals which aresuitable for presentation to an input of the microprocessor. Inparticular, the analog interface circuit, by use of an analog-to-digital(A/D) converter or the like, converts the analog signals generated bythe sensors into digital signals for use by the microprocessor. Itshould be appreciated that the A/D converter may be embodied as adiscrete device or number of devices, or may be integrated into themicroprocessor. It should also be appreciated that if any one or more ofthe sensors associated with the stand mixer 10 generate a digital outputsignal, the analog interface circuit may be bypassed.

Similarly, the analog interface circuit converts signals from themicroprocessor into output signals which are suitable for presentationto the electrically-controlled components associated with the standmixer 10 (e.g., the driver circuit 54). In particular, the analoginterface circuit, by use of a digital-to-analog (D/A) converter or thelike, converts the digital signals generated by the microprocessor intoanalog signals for use by the electronically-controlled componentsassociated with the stand mixer 10. It should be appreciated that,similar to the A/D converter described above, the D/A converter may beembodied as a discrete device or number of devices, or may be integratedinto the microprocessor. It should also be appreciated that if any oneor more of the electronically-controlled components associated with thestand mixer 10 operate on a digital input signal, the analog interfacecircuit may be bypassed.

Thus, the electronic controller 50 may control operation of the motor18. In particular, the electronic controller 50 executes a routineincluding, amongst other things, a control scheme in which theelectronic controller 50 monitors outputs of the sensors associated withthe stand mixer 10 to control the inputs to theelectronically-controlled components associated therewith. To do so, theelectronic controller 50 communicates with the sensors associated withthe stand mixer 10 to determine, amongst numerous other things, theinput current of the motor 18 and/or the rotational speed of the shaft60 of the motor 18. Armed with this data, the electronic controller 50performs numerous calculations, either continuously or intermittently,including looking up values in preprogrammed tables, in order to executealgorithms to generate a motor control signal and provide this signal tothe driver circuit 54.

Referring now to FIG. 4, an illustrative embodiment of a method ofcontrolling the motor 18 using the motor controls 20 of FIG. 3 isillustrated as a simplified flow diagram. The process 100 may beexecuted by the electronic controller 50 with inputs from the othercomponents of the stand mixer 10, as described above. The process 100will generally be performed iteratively, or cyclically, by theelectronic controller 50. In some embodiments, the electronic controller50 may complete the process 100 within each period of the periodic drivesignal generated by the driver circuit 54. For instance, where thefrequency of the periodic drive signal is 15.6 kHz, the electroniccontroller 50 may complete the process 100 in fewer than 64 microsecondsduring each cycle of the periodic drive signal. It is contemplated thatperiodic drive signals having other frequencies may alternatively beused. The process 100 includes a number of process steps 102-112, asshown in FIG. 4.

Prior to the electronic controller 50 executing the process 100, a userwill place foodstuffs in the mixer bowl 26 of the stand mixer 10, attacha mixing element 24, and slide the control knob 22 from “0” to a desiredsetting (e.g., “Setting 3”). The electronic controller 50 will receivean input signal from the user control 22 indicative of the desiredsetting. The electronic controller 50 will begin generating a motorcontrol signal corresponding to the desired setting and provide thismotor control signal to the driver circuit 54. The driver circuit 54will, in turn, generate a periodic drive signal to drive the motor 18such that the rotational speed of the shaft 60 of motor 18 correspondsto the desired setting.

The rotational speed with which the motor 18 operates the shaft 60 (and,hence, the mixing element 24), however, will vary with both the periodicdrive signal and the load applied to the motor 18. The generalrelationship between motor speed (i.e., rotational speed of the shaft60) and motor current for the motor 18 is illustrated graphically inFIG. 5. By way of example, a large batch of cookie dough mixed at“Setting 3” may initially draw an input current of approximately 7 amps(point 126 in FIG. 5). As the mixing element 24 encounters the cookiedough in mixer bowl 26, an increasing load will be applied to the motor18 (e.g., at one of the pinch points 36-41 shown in FIG. 2), and therotational speed of the shaft 60 may slow below the desired speedsetting. Increasing the average current delivered by the periodic drivesignal may achieve the desired speed, despite load variations, but mayalso risk damaging the motor 18. To simultaneously manage both motorspeed and motor current, the electronic controller 50 may executeprocess 100.

The process 100 begins with process step 102, in which the electroniccontroller 50 measures the rotational speed of the shaft 60 of motor 18and measures an input current supplied to the motor 18. The electroniccontroller 50 measures these quantities, respectively, by receiving aspeed signal from the RPM sensor 62 that is indicative of the rotationalspeed of the shaft 60 and by receiving a current signal from the currentsensor 56 that is indicative of the input current supplied to the motor18. In some embodiments, the RPM sensor 62 and the current sensor 56both generate these analog signals and provide them to the electroniccontroller 50 on a continuous basis. In such embodiments, the analoginterface circuit of the electronic controller 50 will convert thesesignals into digital values, as described above. In particular, an A/Dconverter of the electronic controller 50 is operable to sample thesesignals at a predetermined sampling rate. The respective sampling ratesfor the speed signal and the current signal (which may or may not beequal) may have a greater frequency than the periodic drive signalgenerated by the driver circuit 54. This greater frequency allows thesignals to be sampled, and the process 100 to be executed, at least onceduring each period of the periodic drive signal, as described above.

After process step 102, the process 100 proceeds to process step 104, inwhich the electronic controller 50 determines a current limit based onthe rotational speed of the shaft 60 using a substantially continuousfunction which relates a domain of rotational speeds to a range ofcurrent limits. What is meant herein by the term “substantiallycontinuous function” is a function for which the following relationshipgenerally holds: the output of the function as the input approaches avalue is equal to the output of the function at that value, for allinputs in the domain of the function. In other words, relatively smallchanges in the input (rotational speed) result in relatively smallchanges in the output (current limit). In contrast, a step-wise functionis a discontinuous function, not a substantially continuous function.One embodiment of a substantially continuous function 120 relatingrotational speeds of the shaft 60 to corresponding current limits overthe domain of operating speeds of the stand mixer 10 (“Setting 1” to“Setting 10”) is shown in FIG. 5.

In process step 104, the electronic controller 50 calculates a currentlimit based on the rotational speed measured in process step 102. Insome embodiments, the electronic controller 50 uses a substantiallycontinuous function 120 having the form of a linear function 120, asshown in FIG. 5. In other embodiments, the substantially continuousfunction may have the form of a non-linear function. In either of thesecases, the electronic controller 50 may calculate a current limit as theoutput of the function with the measured rotational speed as the input.In still other embodiments, the electronic controller 50 may retrieve acurrent limit value corresponding to the measured rotational speed froma preprogrammed look-up table approximating a continuous function. Alook-up table must have a sufficiently large number of data points to bea substantially continuous function (i.e. changes in rotational speedthroughout the domain must appear smooth, not jerky, to a user of thestand mixer 10). Returning to the example of mixing a large batch ofcookie dough mixed at “Setting 3,” the electronic controller 50 woulddetermine the current limit to be point 122 on the function 120, asshown in FIG. 5.

After process step 104, the process 100 proceeds to process step 106, inwhich the electronic controller 50 determines whether the input currentmeasured in process step 102 has exceeded the current limit determinedin process step 104. In some embodiments, this comparison may beperformed with the comparator 58, where V_(ref) has been set by theelectronic controller 50 to represent the current limit for the desiredspeed setting. In such cases, the output of the comparator 58 willsignal the electronic controller 50 when the stand mixer 10 should enterthe current limitation mode represented by process steps 110-112. If themeasured input current is less than the determined current limit, theprocess 100 instead proceeds to process step 108, in which theelectronic controller 50 regulates the speed of motor 18 according to aproportional-integral-derivative (PID) algorithm, after which theprocess 100 ends. If the measured input current is greater than or equalto the determined current limit, however, the process 100 proceeds toprocess step 110, which is discussed in more detail below.

If the process 100 proceeds to process step 108, the electroniccontroller 50 determines the periodic drive signal which should begenerated by the driver circuit 54 to achieve the desired setting (e.g.,“Setting 3”) using a known PID algorithm. A PID algorithm typicallyutilizes an “error signal” (the desired speed setting less the measuredrotational speed), the integral of the error signal, and the derivativeof the error signal, to calculate the appropriate periodic drive signalwith which to drive the motor 18. So long as the input current suppliedto the motor 18 remains below the current limit (point 122 in the“Setting 3” example), the periodic drive signal can be freely adjustedby the PID algorithm to maintain the desired setting. Thus, in the“Setting 3” example, the operating point of the motor 18 may shiftbetween point 126 (smaller load) and point 128 (larger load) to maintaina certain speed. In some embodiments of the process 100 (now shown),process step 108 may be further stratified to apply different types ofPID algorithms depending on varying input current thresholds. After theperiodic drive signal is (potentially) adjusted in process step 108, theprocess 100 ends and begins again at process step 102.

Once again, if the input current measured in process step 102 is greaterthan or equal to the current limit determined in process step 104,process step 106 will direct the process 100 to proceed to process step110 (rather than process step 108). By way of example, mixing a batch ofpotatoes at “Setting 8” may cause the input current supplied to themotor 18 to approach or meet the current limit determined by theelectronic controller 50 in process step 104 (point 124 on the function120, for this example). Various points 36, 42, 44, 46, and 48 in thecomplex rotational motion of the dough hook 24 (as shown in FIG. 2)generally correspond, respectively, to various points 136, 142, 144,146, and 148 in the relationship between motor speed and motor currentshown in FIG. 5. For example, at point 42 in FIG. 2 (in which arelatively small load is placed on the motor 18), the motor speed willremain substantially at “Setting 8,” but the input current supplied tothe motor 18 will increase toward the current limit 124 as shown bypoint 142 in FIG. 5. As the dough hook 24 approaches point 44 in FIG. 2,the input current will reach (and begin to exceed) the current limit 124as shown by point 144 in FIG. 5. In response to the input currentreaching the current limit 124, the process 100 will proceed to processstep 110.

In process step 110, the electronic controller 50 determines a newperiodic drive signal which should be generated by the driver circuit 54in accordance with the substantially continuous function 120. Using thesubstantially continuous function 120 to determine the new periodicdrive signal will typically result in a slight or incremental, ratherthan significant, reduction in the average current of the periodic drivesignal. The rotational speed of the shaft 60 will not be reduced to thenext lowest setting of the user control 22 (e.g., “Setting 7”) but,rather, some intermediate value, as shown in FIG. 5. This mode ofoperation provides for a smooth, rather than jerky, speed transitionsfor the motor 18. For instance, when the input current reaches the valuecorresponding to point 144 (exceeding the current limit 124), theelectronic controller 50 will calculate a new periodic drive signal withan average current cycle designed to operate the motor 18 at point 144on the substantially continuous function 120, as shown in FIG. 5. Theelectronic controller 50 modifies the motor control signal it generatesand supplies to the driver circuit 54 in order to cause this change inthe periodic drive signal.

After process step 110, the process 100 proceeds to process step 112, inwhich the newly determined periodic drive signal is generated by thedriver circuit 54 and drives the motor 18. As just discussed, the newperiodic drive signal should result in an incremental reduction in therotational speed of the shaft 60. After process step 112, the process100 ends and begins again at process step 102. Until the input currentsupplied to the motor 18 returns below the original current limit 124that was set as V_(ref) at the comparator 58, the process 100 willremain in the current limitation mode and iteratively cycle throughprocess steps 102-106 and 110-112. This current limitation mode may beused to protect the motor 18 from the intermittent or instantaneousloading that accompanies the pinch points 36-41 discussed above withreference to FIG. 2.

During the pass through the process 100 just discussed and shown in FIG.5, the operational point of the motor 18 was moved to point 144 bymodifying the periodic drive signal, in response to determining that theinput current at point 144 exceeded the current limit 124, to achievethe incrementally lower speed on the function 120. As the input currentcontinues to increase (due to increased loading of the motor 18 as thedough hook 24 approaches the pinch point 36), the process 100 willrepeat and the average current of the periodic drive signal will againbe incrementally decreased. This will continue until the pinch point 36(i.e., the maximum load on the motor 18) has been reached and the inputcurrent ceases to increase. This operational point of the motor 18 isrepresented as point 136 on the substantially continuous function 120 inFIG. 5.

As the dough hook 24 moves away from the pinch point 36, the load on themotor 18 will decrease, causing the input current supplied to the motor18 to decrease below the determined current limit. During these passesthrough the process 100, the electronic controller 50 will respond byincrementally increasing the average current of the periodic drivesignal and, thus, the rotational speed of the shaft 60 until therotational speed returns to the desired setting chosen by the user. Thisincremental increase in the motor speed, returning toward to “Setting8,” is illustrated as points 146 and 148 in FIG. 5. As discussed above,each pass through the process 100 may be performed by the electroniccontroller 50 within the period of the periodic drive signal used todrive the motor 18. The frequency of these adjustments also contributesto the smooth transitions between motor speeds.

While the disclosure has been illustrated and described in detail in thedrawings and foregoing description, such an illustration and descriptionis to be considered as exemplary and not restrictive in character, itbeing understood that only illustrative embodiments have been shown anddescribed and that all changes and modifications that come within thespirit of the disclosure are desired to be protected. For example, whileportable appliance 10 is herein illustrated as a stand mixer, thefeatures and aspects disclosed herein can also be implemented in othertypes of portable appliances, such as hand mixers, blenders, immersionblenders, juicers, food processors, and the like. It is alsocontemplated that the systems and methods of the present disclosure maybe applied to motor control in any type of appliance (for example,washers, dryers, refrigerators, freezers, etcetera).

Furthermore, embodiments of the disclosed systems and methods may beimplemented in hardware, firmware, software, or any combination thereof.Embodiments of the disclosed systems and methods may also be implementedas instructions stored on a tangible, machine-readable medium, such asthe memory device 52, which may be read and executed by one or moreelectronic controllers 50. A tangible, machine-readable medium mayinclude any mechanism for storing or transmitting information in a formreadable by a machine (e.g., an electronic controller 50). For example,a tangible, machine-readable medium may include read only memory (ROM),random access memory (RAM), magnetic disk storage, optical storage,flash memory, and/or other types of memory devices.

There are a plurality of advantages of the present disclosure arisingfrom the various features of the systems and methods described herein.It will be noted that alternative embodiments of the systems and methodsof the present disclosure may not include all of the features describedyet still benefit from at least some of the advantages of such features.Those of ordinary skill in the art may readily devise their ownimplementations of the systems and methods that incorporate one or moreof the features of the present invention and fall within the spirit andscope of the present disclosure as defined by the appended claims.

What is claimed is:
 1. A method of controlling a portable appliance, themethod comprising: providing a user control that is configured to permita user to input a desired revolutions-per-minute (RPM); measuring aninput current supplied to an electric motor of the portable applianceutilizing a current sensor; measuring an RPM of a shaft of the electricmotor utilizing an RPM sensor; using the measured RPM of the shaft todetermine a current limit based on a predefined substantially continuousfunction which defines input current limits as a function of shaft RPM;if the measured input current does not exceed the current limit for themeasured RPM of the shaft, using a constant RPM mode in which the inputcurrent supplied to the electric motor is actively varied to maintainthe RPM of the shaft at a desired RPM that has been input by a user; ifthe measured input current exceeds the allowable current limit, using acurrent limitation mode in which the RPM of the shaft is reduced belowthe desired RPM that has been input by a user and the electric currentis increased to the current limit while the RPM is reduced, such thatthe RPM of the shaft is varied according to the substantially continuousfunction and the current is maintained at the current limit according tothe substantially continuous function.
 2. The method of claim 1,wherein: the user control provides a finite number of possible discreetRPM settings; when in the current limitation mode, causing the electricmotor to operate at an RPM that is less than a desired RPM set by a userbut greater than the next lowest RPM settings.
 3. The method of claim 1,including: utilizing a controller to calculate an electrical currentlimit using the measured RPM as the input.
 4. The method of claim 3,wherein: the controller does not take into account a desired RPM settingwhen operating in the current limitation mode.
 5. The method of claim 3,wherein: the controller utilizes a PID control when in the constant RPMmode.
 6. The method of claim 5, wherein: the controller causes the RPMof the shaft to decrease below the desired RPM and increase back to thedesired RPM in a periodic manner due to periodic variations in loadingon the electric motor.
 7. The method of claim 2, wherein: the samesubstantially continuous function is utilized for all possible RPMsettings.
 8. The method of claim 1, wherein: measuring the input currentsupplied to the electric motor comprises periodically sampling, at afirst sampling rate, an output signal of the current sensor; measuringthe RPM of the shaft of the electric motor comprises periodicallysampling, at a second sampling rate, an output signal of the RPM sensor;and the first and second sampling rates each have a greater frequencythan a periodic drive signal used to drive the electric motor.
 9. Themethod of claim 1, including: using an electronic controller todetermine the current limit using the substantially continuous functionby calculating the output of a linear function using the measured RPM ofthe shaft as the input to the substantially continuous function.
 10. Themethod of claim 1, including: using an electronic controller todetermine the current limit using the substantially continuous functionby calculating the output of a non-linear function using the measuredRPM of the shaft as the input to the substantially continuous function.11. The method of claim 1, including: using an electronic controller todetermine the current limit using the substantially continuous functionby retrieving a value which corresponds to the measured RPM of the shaftfrom a look-up table.
 12. The method of claim 1, including: using anelectronic controller to reduce the RPM of the shaft along thesubstantially continuous function while increasing current supplied tothe electric motor by, iteratively (i) reducing the RPM of the shaft byan increment, (ii) measuring a new RPM of the shaft, (iii) measuring anew input current, and (iv) determining a new current limit based on thenew measured RPM of the shaft using the substantially continuousfunction, until the new input current is approximately equal to the newcurrent limit.
 13. A mixer comprising: a user control operable togenerate an input signal indicative of a desired RPM setting for themixer; an electric motor having a shaft configured to provide motivepower to a mixing element, the electric motor defining a range ofnonequal current limits that, if exceeded, damage the electric motor; acurrent sensor configured to measure an input current supplied to theelectric motor; a revolutions-per-minute (RPM) sensor configured tomeasure RPM of the electric motor; an electronic controller configuredto (i) generate a motor control signal such that the RPM of the electricmotor is maintained at the desired RPM speed setting when the controlleris in a constant RPM mode, (ii) calculate a current limit based onmeasured RPM using a predefined substantially continuous function whichrelates RPM of the electric motor to a range of nonequal current limitssuch that each RPM is associated with a specific predefined currentlimit, and wherein the RPM of the substantially continuous functiondecreases with increasing electrical current limit, and (iii) use acurrent limitation mode in which the motor control signal is modified,when the input current exceeds the current limit for the measured RPMaccording to the substantially continuous function, such that, when theelectronic controller is in the current limitation mode, the RPM of theelectric motor is reduced according to the substantially continuousfunction while the electrical current to the electric motor ismaintained at a level that is approximately equal to the current limitfor the measured RPM.
 14. The mixer of claim 13, wherein: the currentsignal exceeds the current limit when a pinch point occurs between themixing element and a mixer bowl that receives the mixing element. 15.The mixer of claim 13, wherein: the substantially continuous functioncomprises one of a linear function, a non-linear function, and a look-uptable.
 16. The mixer of claim 13, further comprising: a driver circuitoperable to generate a periodic drive signal to drive the electric motorin response to the motor control signal.
 17. The mixer of claim 13,wherein: the user control includes a finite number of desired RPMsettings; and the electronic controller is configured to cause theelectric motor to operate at RPMs between adjacent RPM settings when inthe current limitation mode.